it seems turbocache cannot be disabled:
it should really be time that nvidia start looking seriously into this issue.
I did some test casing as described in all the other threads and it all comes down to 2.6.19 => 2.6.20.
I'd have gladly do regression testing, and test each 2.6.20-rc* but I can't find them, or even each git commit to trace the culprit commit but I don't know how to do that. and I don't feel like I have to do nvidia's work...
IIRC there have been some pci and memory management changes in 2.6.20, and I think that's the cause, but who knows.
(see eg under various core changes)